dup Assignment#create API docs to #update
dup Assignment#create API docs to #update

The Assignment#update API method needs to have arguments so that
the auto-generated Pandarus gem can know what to send to Canvas.

Test Plan:
- Docs for "Edit an assignment" should have same args as "Create
  an assignment"

Fixes SIS-314

Change-Id: If50e40ee345a6a41b8a45368ed77b65f1b364e80
Reviewed-on: https://gerrit.instructure.com/35942
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Product-Review: Eric Adams <5108e8d39c3daf832a5d5b01c70727c22a362224@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>

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